using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var N = int.Parse(Console.ReadLine());
        var set = new HashSet<int>();

        int[] input;
        var result = 0;
        for (int i = 0; i < N; i++)
        {
            input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            result = input[1] - input[0];
            if (result >= 1) set.Add(result);
        }
        if (set.Count() == 0) Console.WriteLine(-1);
        else
        {
            var count = set.Distinct().Count();
            var ans = set.First();
            Console.WriteLine((count == 1) ? ans : -1);
        }
    }
}